My dear Faust
by loosingletters
Summary: Rin had always thought it was ironic that his adopted father wasn't the demon. Weird as hell fashion sense, tons of familiars, a love for game and bets and a vast knowledge of how the universe worked - Johann Faust was the weirdest human there was. Then again, Shirou Fujimoto was the weirdest demon there was. It fit somehow.


**Summary: Rin had always thought it was ironic that his adopted father wasn't the demon. Weird as hell fashion sense, tons of familiars, a love for game and bets and a vast knowledge- Johann Faust was the weirdest exorcist there was.**

 **A Role reversal AU with Mephisto as the human Johann Faust and Shirou as the Demon King of Time!  
**  
At eight years old, the earliest memory Rin could recall was one of Yukio crying. They couldn't have been much older than three and Rin was sitting on the sofa as Papa picked Yukio up. He told him of Gehenna and Assiah, about each of the familiars serving in the household and about the missions he had been on.

Sadly Yukio kept on crying because such stories didn't calm a toddler when they had just hurt themselves.

The next thing Rin recalled was somebody, who had been sitting next to him, getting up and taking Yukio out of Papa's hands with a sigh. The man wasn't as tall as Papa but close. He had white hair and piercing red eyes. He wore black clothes Rin now knew were the exorcist uniform. The attire clashed totally with Papa's white or pink clothes. Rin knew only one person who appeared to be a total opposite of his father while at the same time fitting the man so well.

Fujimoto Shirou.  
 _Samael._  
Demon King of Time.

Shirou was often around, even though he tried to keep his distance. Somehow he never managed though, probably fearing that Papa would end up accidentally drowning Rin and Yukio or revealing their nature to the world. Rin honestly doubted that Papa would ever make such a mistake, he was brilliant.

Still-

Papa and Shirou fought about Rin and Yukio all the time.

Papa thought it would be stupid to try to make them human when they clearly weren't. Rin agreed. He liked the way he was, he didn't particularly want to be human. Not when Papa told him how great Yukio and he could be one day if they gave it their all. Not when he told them about all the demons that would bow to them. But even more than that would Rin miss using his flames for making smores when they sat outside on warm summer nights. Rin was sure that Yukio would also never want to give up his abilities. Yukio had amazing perception and the way his blue eyes would shine with the same color as Rin's flames was just _awesome_.

Rin would never deny his demonic heritage, even though it was scary at times and forced Yukio and him to be very careful when in proximity to people, especially exorcist. But still, it was a part of him. Rejecting it would do him no good (and living without a tail would be way too weird).

Shirou disagreed with Papa on that topic. He called Yukio and Rin _innocent_ as if that had something to do with the fact that they had human blood too. Shirou wanted them out of the war he was fighting.

"Two siblings less to worry for," he had said to Papa. "Keep them out of it, Johann."

Papa never listened either way and Rin was thankful for it. One of Papa's summons - or at least Rin thought it was a summon - was Amaimon, the Earth King. Often also called Ambrosius and posing as Papa's little brother. That particular actuality made Rin frown. It was _weird_. Amaimon was Yukio and Rin's older brother after all, right? Just like Shirou! If he was pretending to be Papa's brother, he'd be Rin and Yukio's uncle and that was just wrong.

Last time Amaimon came to visit while Shirou was there, the younger demon got the scolding of his life. It didn't seem to bother him as much though.

"Nii-san's always been like that," Amaimon had told Yukio and Rin. "He worries and worries and lectures and lectures and tries to stop us from causing chaos. It doesn't work though."

After that Amaimon, whose range of expressions was somewhere between nonexistent and boredom, looked almost hurt.

"That's why he wants to keep you out, wants you to be human. You're less likely to become as messed up as us then."

Yukio had hugged Amaimon then. It had looked awkward and rather funny, it was no wonder Papa had joined the hug. Papa joining the hug looked right too. Rin couldn't imagine what it would have looked like if Shirou had joined in. Wrong probably. Shirou was almost too normal to look right in their household. Papa and Amaimon though could almost pass as related. They possessed the same kind of _weird_. Yukio had told him that it wasn't kind to call people weird but Rin didn't particularly care. People called Papa lots of fancy words when what they really meant was weird.

Eccentric for example was what other people called Papa. Reckless too. And Irresponsible. But the insult that came up the most when people thought they were being silent was _Demon lover_.

That one always made Papa laugh.

"They're right, you know!" He had told them. "I truly love demons, more than humans even! I love you, my little hellspawns and Amaimon-"

"Shirou too?" Yukio had asked, blue eyes full of wonder.

"Oh, yes. Him especially."

Papa had smiled kindly. It was all teeth and madness.

"Did I ever tell you the story of Johann Georg Faust?"

The twins had stared at him, lack of comprehension shining in their blue eyes.

"Your story?" Yukio asked.

"In a way," Papa replied. "Long ago there lived a man by that name and he made a pact with a demon that back then was called Samael. The demon accompanied Faust, who was always greedy for knowledge, for 24 years. It was Faust who made Samael bet with him over and over again, selling away more and more each time. As human as Faust was, he certainly loved demons and their deals. So for a long time they lived side by side until Faust, thinking he could do greater than Nicholas Flamel or King Solomon and become a demon himself, made a mistake and paid dearly. As Faust was dying, Samael devoured Faust's soul as a payment for the 24 years he had helped the man."

Papa took a break and poured himself another cup of tea before he continued.

"But the demon, and wasn't that the miracle, didn't let the soul dissolve to strengthen himself! Instead he kept it and watched it. He watched the soul of Faust who wanted to become Mephistopheles so much. The legends say that Samael was a demon that had come to enjoy Assiah. And for him Faust, whose own fate he had bet against heaven, had to exist on Assiah. Some people say that it was love what the demon had learnt on Assiah but that's ridiculous. If he could already love, he wouldn't try so desperately to be human, right?"

Rin and Yukio shared a glance. Both of them couldn't really follow their Papa. That was rather often the case. It would take ages until the twins comprehended another carefully worded explanation or riddle their Papa had given them.

"And since Samael had come to care for Faust," Papa continued, "he broke time just for Faust instead. So whenever he saw the opportunity, he brought Faust back to life. In exchange for that being overlooked, Faust always had to stay in the Vatican's care and work for them alongside with the King of Time."

With that Papa ended his tale and took two small cups off the tray and poured tea into them too. Rin smelled cherries, haws and banana – children's tea for him and Yukio.

"Not that I mind. The most interesting things happen here," Papa said and smiled. "And what would I be without Shirou?"

Rin was left with more questions than answers.

 **I'm trying to write Mephisto x Shirou actually but it doesn't work. I end up with weird as hell Aus like this eeh. No idea if I'll write more for this AU. Probably though cause Mephisto/Johann being the human dad of the twins is hilarious. Also I tweaked the Faust story. Instead of Faust as the one being played, he bets away his own life knowingly as the demon watches.**

 **Thanks for reading! I'd love to hear what you think!**


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